The production capacity of ethanol from corn in the United States reached 9 billion gallons per year in 2008. The use of ethanol as gasoline additive not only reduces the emission of harmful air pollutants, but also lowers dependence on imported fossil fuels. About six billion gallons of the ethanol produced in 2008 was in dry-grind corn milling plants and this output is set to almost double before 2011 with the capacity currently under construction. Dry-grind corn milling involves milling, cooking, enzyme addition and fermentation by yeasts in water. During fermentation in a corn dry milling facility, only 30-35% of the corn is actually converted to ethanol, about one-third to CO2, and one-third remains as dissolved organics and suspended solids in the (whole) stillage after ethanol removal by distillation in distillation columns. The stillage contains about 89% water.
In a traditional dry-grind ethanol production process more than 75% of the solids in stillage are removed by centrifugation. This solids fraction, known as thick stillage is dried to a product known as distillers dried grains (DDG). Dry-grind corn ethanol plants produced 23.0 million metric tons of distillers grains in 2008, a considerable increase from the 14.6 million metric tons in 2007. The excess centrate, known as thin stillage, is evaporated to produce syrup, which is usually added to the centrifuged solids prior to drying. The dried product from this combination is known as DDG with solubles (DDGS), and is often sold as animal feed. DDGS are low in essential amino acids, particularly lysine (about 0.7%), and methionine (about 0.3%). This limits the use of DDGS in animal feed to part of the ruminant diet mainly, while the demand for feed for hogs and chickens, particularly in the corn belt states, is much larger, DDGS are largely unsuitable as feed for monogastric animals.
The excess water from the centrifuge (centrate), or thin stillage, contains about 3 to 4% suspended solids and 2 to 5% by mass dissolved organic materials. The total content of dissolved and suspended organic materials can be typified by measurement of the oxidizable material and expressed as the amount of oxygen equivalents needed for this oxidation. This is known as the chemical oxidant demand (COD) and the typical value of COD in thin stillage is 100 g/L. This organic material could be available for value-added products.
Disposal of the stream of thin stillage by evaporation, while only recovering a low-value syrup, is a major burden on ethanol plants. Further, the use of syrup in animal nutrition is questionable—with many nutritionists and feeders preferring DDG to DDGS, that is byproduct without solubles. Also, if the distillers grains are not dried, there is no opportunity to combine the syrup with the grains. As a result, many plants end up burning the syrup for some energy recovery or giving it away.
In the ethanol production process, it is a desirable and common practice to recycle thin stillage to the cookers prior to introduction to the fermentors to reduce the need for evaporation to syrup. A maximum of 50% of the thin stillage can be recycled by this mechanism to prevent build-up of inert materials and fermentation byproducts. Fermentation byproducts are essentially waste substances of fermentation and are inhibitory to yeast fermentation. It should be appreciated that most of the organic materials left after fermentation are not amenable to yeast fermentation. The main undesirable components in thin stillage for recycling are the solids and the fermentation byproducts glycerol, lactic acid and acetic acid, which all occur in appreciable concentrations in thin stillage. Recycling of the inert materials and/or fermentation byproducts also results in adding bulk and providing nutrients for unwanted bacterial growths.
An ethanol plant needs about 5 to 6 gallons of water to be added into the fermentation process per gallon of ethanol of produced. Almost half of this is water obtained from the up to 50% recycle of thin stillage. The remainder of the water must be obtained by other means. In other words, additional water must be added to the process. A typical 100 million gallon per year ethanol plant requires 300 million gallons or more of fermentation “make-up water” plus another 120 million gallons per year for other purposes. Most of this water is evaporated as these plants discharge almost no wastewater. As a consequence, at least 500 gallons thin stillage per minute needs to be evaporated on a 100 million gallon/year ethanol plant—adding up to as much as 25 billion gallons per year industry wide. Evaporation is costly and adds about $0.10 per gallon to the cost of producing ethanol (2008 costs), uses non-renewable energy (natural gas usually) and it releases volatile air pollutants. Although heat recovery from evaporation lowers the energy requirements, thin stillage evaporators represent a bottleneck in expanding production on existing ethanol plants. Current plants attempt to lower the evaporation cost through heat recovery from water. Some plants condense the water, but the condensate is rich in volatile organic acids that necessitate additional treatment (such as anaerobic bacterial treatment in a methanator) for organic removal before the water can be reused as process water.
As discussed, in the typical ethanol production process the non-volatile organic material in the thin stillage is concentrated into a low-value syrup. This organic material, along with the volatile organic compounds lost during evaporation, represents a resource that could be converted into more valuable co-products with a higher value. There are other low-value streams generated in other crop-processing industries such as the stillage from fermentation of other grains, sugar cane, agave plants or molasses to ethanol, starch or sugar wastes, stillage from the production of other alcohols and also the whey left-over in soy processing. These streams all represent underutilized resources that could be used in generating more valuable co-products.
Ethanol production is still growing rapidly. As already noted, ethanol is produced mainly by dry-grind corn milling, co-producing considerable amounts of DDG and low-value thin stillage. Profitability in the ethanol industry is threatened by corn prices rising due to increased demand, while DDG may flood the market. In addition, it is to be expected that input energy costs will continue to increase as was demonstrated by the huge spike in oil and natural gas cost in mid-2008. Thus, the industry needs to adapt. The present invention is directed to value adding to an ethanol production process through providing additional co-products, extending marketability, reducing external enzyme needs and saving energy by eliminating the need to evaporate water from thin stillage.